Light emitting diodes (LED) have reached performance levels that enable such LEDs to be utilized in applications that were not previously possible, such as industrial and consumer lighting applications in which incandescent and fluorescent lighting systems have typically been utilized for many years. When used in these industrial and consumer applications, LED lighting systems ideally will be easily interchangeable with these prior lighting systems to gain acceptance and utilization in these types of applications. For example, these prior lighting systems receive power from alternating current (AC) power sources and provide some level of power factor correction such that the lighting system effectively presents a resistive load to the power source. LED lighting systems should also be operable from AC power sources and provide the desired power factor correction.
In contrast to conventional lighting systems, however, LED lighting systems require a constant current be supplied through the LEDs to provide the desired illumination. Typically a large number of LEDs are connected in series and parallel combinations to provide the desired illumination. A variety of different types of voltage converters have been utilized in prior systems to drive LED lighting systems in the required manner and thereby provide the required constant current to achieve the desired illumination. FIG. 1 is a circuit diagram showing a conventional LED drive circuit that is formed by a synchronous Buck converter drive circuit 100 for converting an input voltage Vin into an output voltage Vout desired for driving one or more series-connected LEDs 102.
In operation, an inductor current IL1 flows through an inductor L1 when a first switching transistor Q1 is turned ON and a second switching transistor is turned OFF. A switching control circuit 104 applies drive controls signals DCS1 and DCS2 to control the activation and deactivation of switching transistors Q1 and Q2. The switching control circuit 104 drives the DCS1 signal active and the DCS2 signal inactive to turn the transistor Q1 on and the transistor Q2 OFF. During this mode of operation, the current IL1 flows through the inductor L1 and charges a load or output capacitor COUT to develop an output voltage VOUT across the capacitor and thereby across the series-connected LEDs 102.
During a second mode of operation, the control circuit 104 deactivates DCS1 and activates DCS2, turning the transistors Q1 and Q2 OFF and ON, respectively. In this mode, with the transistor Q1 turned OFF and Q2 turned ON the voltage developed across the inductor L1 supplies current through the transistor Q2 to maintain the current IL1 through the inductor L. The conventional operation of the Buck converter drive circuit 100 is well understood by those skilled in the art and thus, for the sake of brevity, will not be described in more detail herein.
The control circuit 104 pulse width modulates the DCS1 and DCS2 to define a duty cycle D for the transistor Q1, with the duty cycle being defined by an on-time TON corresponding to the duration of a period T of the DCS1 signal for which the transistor is turned ON. More specifically, the duty cycle D is given by D=TON/T. The voltage developed across the output capacitor COUT corresponds to the output voltage VOUT from the drive circuit 100 and an output current IOUT from the output capacitor drives the series-connected LEDs 102 to provide current through these LEDs to achieve the desired illumination intensity.
A current transducer 106 is connected in series with the LEDs 102 and functions to generate a feedback voltage signal VFB having a value that is a function of the output current IOUT flowing through the series-connected LEDs 102. The control circuit 104 receives the feedback voltage signal VFB and utilizes this signal in generating the pulse width modulated signals DCS1 and DCS2 to control the duty cycle D of the transistors Q1 and Q2 and the overall operation of the Buck converter drive circuit 100. The feedback voltage VFB has a value that is a function of the current IOUT through the LEDs 102 and in this way enables the switching control circuit 104 to control this current. In this way, the current transducer 106 directly senses the current flowing through the series-connected LEDs 102. With the approach of FIG. 1, a suitable current transducer 106, such as a sense resistor or Hall Effect device, is utilized to sense the output current IOUT. The current transducer 106 increases the parts count of the Buck converter drive circuit 100, which increases the size and cost of the drive circuit.
There is a need for improved driver circuits and methods for controlling LED lighting systems.